sp-api: Move macro related re-exports to __private (#2446)

This moves the macro related re-exports to `__private` to make it more
obvious for downstream users that they are using an internal api.

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Co-authored-by: command-bot <>

substrate/utils/frame/benchmarking-cli/src/storage/write.rs

@@ -18,10 +18,10 @@
 use sc_cli::Result;
 use sc_client_api::{Backend as ClientBackend, StorageProvider, UsageProvider};
 use sc_client_db::{DbHash, DbState, DbStateBuilder};
-use sp_api::StateBackend;
 use sp_blockchain::HeaderBackend;
 use sp_database::{ColumnId, Transaction};
 use sp_runtime::traits::{Block as BlockT, HashingFor, Header as HeaderT};
+use sp_state_machine::Backend as StateBackend;
 use sp_trie::PrefixedMemoryDB;
 
 use log::{info, trace};